Abstract: An injection molding system is provided with a plurality of wall segments which are displaceable substantially radially for forming a substantially continuous wall which forms an internal wall of a hollow region which is intended to be filled with molten molding material. In a preferred embodiment, the wall segments are urged into sliding communication with one another and sealable engagement with a further wall portion by operation of an actuator assembly which is provided with canted pins which cause the wall segments to be moved radially inwardly and outwardly in response to orthogonal displacement of the actuator assembly. The invention is therefore particularly suited for forming molded articles having hollow interiors and/or internally ribbed portions. In an embodiment where a slot is desired in the molded article, a slot core is arranged to communicate with the molten molding material, so as to form the slot, and an ejection mechanism.